An amino acid derivative in which a hydrogen atom in an amino group of amino acid is substituted or protected by an alkoxycarbonyl group such as tert-butoxycarbonyl, an aralkyloxycarbonyl group, an aryloxycarbonyl group, an alkenyloxycarbonyl group or an acyl group is an important compound as a medicine or a medicinal intermediate. In these amino acid derivatives, optical isomers are present in many cases, and it is widely known that a large difference in a physiological activity and a toxicity thereof is frequently present between these optical isomers. Accordingly, it is very important in amino acid derivatives used in such fields to produce the products having a high optical purity and confirm the optical purity prior to use thereof.
In general, optical isomers can be separated and refined by means of liquid chromatography, and various columns for separating optical isomers in which polysaccharides or amino acid derivatives are carried on silica gel have been developed.
Introduced in, for example, Takeuchi et al., Journal of Chromatography, 357 (1986), p. 409 to 415 is an example in which α-cyclodextrin and β-cyclodextrin are used as a mobile phase component in separating racemic mandelic acid and a racemic mandelic acid derivative, and it is introduced therein that a stationary phase in which cyclodextrin is bonded for high performance (or high speed) liquid chromatography (HPLC) is applied for separating optical isomers, geometrical isomers and structural isomers.
Besides, the authors of the literature have reported separation of enantiomers of dansyl amino acid carried out by micro HPLC in which β-cyclodextrin is used as a mobile phase component. The separating method described in the above publication makes use of the character that β-cyclodextrin is liable to make a complex (clathrate complex) with a low molecular compound having a naphthalene ring which constitutes a dansyl group, and the above separating method can not be applied as it is to separation of optical isomers of an amino acid derivative having no naphthalene ring. For example, it is explained in the above literature that the higher the pH of a mobile phase becomes, the larger the separation factor tends to grow and that the vicinity of pH 6 is the most preferred. However, it would be impossible to separate the optical isomers of the amino acid derivative described above having tert-butoxycarbonyl in such pH region.
An optical isomer of an amino acid derivative in which a dansyl group is introduced into an amino group of amino acid can be separated by the method of Takeuchi et al. described above, but it is not necessarily easy in a certain case to remove the dansyl group and allow the amino acid derivative to be back to the original amino acid. Accordingly, still strongly required to be provided is a method for separating an amino acid derivative which can be used as a synthetic intermediate for medicines, for example, physiologically active peptide and which is a group (refer to, for example, tert-butoxycarbonyl described above) introduced for such use and/or separation and capable of being easily removed after a prescribed purpose is achieved.